CMakePackage: convert variants to CMake arguments (#14376)

Add a 'define_from_variant` helper function to CMake-based Spack
packages to convert package variants into CMake arguments. For
example:

  args.append('-DFOO=%s' % ('ON' if '+foo' in self.spec else 'OFF'))

can be replaced with:

  args.append(self.define_from_variant('foo'))

The following conversions are handled automatically:

* Flag variants will be converted to CMake booleans
* Multivalued variants will be converted to semicolon-separated strings
* Other variant values are converted to CMake string arguments

This also adds a 'define' helper method to convert any variable to
a CMake argument. It has the same conversion rules as
'define_from_variant' (but operates directly on values rather than
requiring the user to supply the name of a package variant).

var/spack/repos/builtin.mock/packages/cmake-client/package.py

@@ -21,6 +21,14 @@ class CmakeClient(CMakePackage):
 
     version('1.0', '4cb3ff35b2472aae70f542116d616e63')
 
+    variant(
+        'multi', description='',
+        values=any_combination_of('up', 'right', 'back').with_default('up')
+    )
+    variant('single', description='', default='blue',
+            values=('blue', 'red', 'green'), multi=False)
+    variant('truthy', description='', default=True)
+
     callback_counter = 0
 
     flipped = False